Reading the CV of one of the teachers at our university I found this line of "My *Erdös number* is 4". I thought - what the hell is an Erdös number? So I asked everything2, but - no entry. I have to do it myself, again. Next, I asked google and found some information - lot's of CVs including Erdös numbers, and finally, the page of the Erdös number project (http://www.acs.oakland.edu/~grossman/erdoshp.html):

To make it short: Erdös numbers seem to be some part of the folklore of mathematicans - Paul Erdös was an hungarian mathematican, who published a lot of papers, and even more with some co-author. The Erdös number is defined as the distance in a graph consisting of mathematicans (vertices) and their co-laborative publications (edges) between one self and Erdös. So the Erdös number of Paul Erdös is 0, the number of everybody who published at least one paper with Paul Erdös is 1, and so on.

It seems that there is a lot of additional theory about Erdös numbers - the subset of all people with finite Erdös numbers, the average Erdös number of Nobel prize winners, and so on ...

**Addendum:**: and of course, Everything2 indeed knows a lot about Pal Erdos and Erdos numbers - how could I dare to include umlauts on the 'o' ...